Enhancement of the ammonia synthesis activity of a Cs- or Ba-promoted ruthenium catalyst supported on barium niobate

Barium niobates with different crystalline structures and morphologies were prepared via a hydrothermal method and applied as a support for a ruthenium catalyst in ammonia synthesis. The sample synthesized with a nominal Ba/Nb ratio = 2.0, having a pure Ba5Nb4O15 crystalline phase and uniform flake-like structure, exhibited the best performance as a support in ammonia synthesis. The flake-like substrate favored the uniform distribution of ruthenium on its surface, which could promote ruthenium to expose more B5 sites. Addition of a Ba- or Cs-promoter enhanced the activity of the Ru/Ba5Nb4O15 catalyst markedly. The highest rate of ammonia synthesis over 2Cs- and 1Ba-4 wt% Ru/Ba5Nb4O15 was 4900 and 3720 (μmol g−1cat h−1) at 0.1 MPa and 623 K, respectively. Both catalysts were stable during the reaction for 72 h at 673 K and 0.1 MPa. Thus, the synthesized Ba5Nb4O15 is expected to be a promising oxide support for ruthenium catalysts for ammonia synthesis.


Introduction
Ammonia plays a very important role in the chemical industry and is oen used in elds such as fertilizers and pharmaceutical intermediates.In recent years, liquid ammonia as a hydrogen-storage material has received widespread attention.Industrial synthesis of ammonia mainly adopts the Haber-Bosch process under high temperature and pressure, and oen uses iron-based catalysts, which causes high energy consumption and considerable pollution. 1Therefore, developing catalysts that can achieve a high performance at low temperature and low pressure is the key to this process.
3][4] The ruthenium-base catalyst requires a suitable support to disperse and support the ruthenium component.At present, electronic compounds, [5][6][7] hydrides, 8 carbon materials [9][10][11] and metal oxides 12,13 are used as supports of ruthenium-based catalysts.Li et al. 14 investigated Nvs-g-C 3 N 4 / graphene as a support.Related studies have shown that electronic compounds and hydrides have good electron-transfer capabilities, but their preparation is complex and difficult to put into practical application.Carbon materials have a large specic surface area but are prone to methanation. 15Metal oxides have attracted the attention of researchers due to their stable structure, simple synthesis, good conductivity and thermal stability.In addition to traditional metal oxides such as MgO [16][17][18] and Al 2 O 3 , [19][20][21] special structures such as BaCeO 3 , 22 Sr 2 Nb 2 O 7 , 23 Mo 2 TiC 2 O 2 (ref.24) and MgAl-LDO (ref.25) have been reported in recent years.
The introduction of promoters can improve the dispersion of Ru, increase the electron density on the surface of Ru, reduce the activation energy of dissociated N 2 , and thereby enhance the activity of ammonia synthesis. 26,27][30][31] We have studied Sr 2 Nb 2 O 7 and Sr 2 Ta 2 O 7 as supports for ruthenium catalysts, which can exhibit high activity at milder reaction conditions.Numerous studies have shown a strong interaction between alkaline oxide carriers and Ru, making them ideal metal-oxide carriers.Ba and Sr belong to the same group of elements, and Ba has stronger alkalinity.Herein, we demonstrate that Ba-or Cs-promoted Ru supported on a barium niobate substrate exhibited superior activity.

Preparation of the Ba 5 Nb 4 O 15 support
The Ba 5 Nb 4 O 15 support was prepared by a hydrothermal method.First, a precursor of niobium (Nb 2 O 5 $nH 2 O) was synthesized from NbCl 5 via a hydrothermal method according to a published report. 32An alcohol solution of NbCl 5 (0.37 mol L −1 ) was prepared by dissolving an appropriate amount of NbCl 5 (99.0%;MilliporeSigma) in ethanol (analytical purity; Sinopharm Chemical Reagents).Then, the pH of the solution was adjusted to ∼10.5 by adding NH 4 OH aqueous solution (4 wt%) with stirring at room temperature for 4 h.Then, a Teon™-lined autoclave was used to hold this mixture, followed by treatment at 473 K for 24 h.Subsequently, a solid product was recovered by centrifugation (8000 rpm for 30 min) and washed repeatedly with pure water until the ltrate was conrmed to be free of chloride anions using AgNO 3 .Finally, the product was dried at 353 K under a vacuum of ∼1.0 kPa.
Second, Ba(OH) 2 $8H 2 O (99.5%; MilliporeSigma) and the resultant Nb 2 O 5 $nH 2 O was dispersed in 45 mL of pure water and stirred for 0.5 h at room temperature.Then, a Teon-lined autoclave was used to hold this mixture, followed by treatment at 473 K for 24 h.The resultant product was ltered and washed with pure water to near neutral pH.The product was dried in a vacuum (∼1.0 kPa) at 353 K for 10 h.

Preparation of the supported Ru catalyst
Ruthenium catalysts were synthesized by an impregnation method.A certain amount of Ru 3 (CO) 12 (99.0%;Milli-poreSigma) was dissolved in approximately 10-25 mL of tetrahydrofuran (THF) solution.Then, dried Ba 5 Nb 4 O 15 (0.20 g) was added.The mixture was stirred at room temperature for 12 h.Then, the solvent was removed using a rotary evaporator (313 K, 10 kPa).Dried samples were decomposed at 723 K for 3 h in an Ar (99.999%) ow of 5 mL min −1 .The catalysts obtained had a nominal Ru loading of 4 wt%.An appropriate amount of CsNO 3 or Ba(NO 3 ) 2 was added to the obtained Cs-or Ba-Ru/ Ba 5 Nb 4 O 15 catalyst by impregnation.

Activity measurements
The activity of the Cs-or Ba-Ru/Ba 5 Nb 4 O 15 catalysts for ammonia synthesis was tested in a xed-bed plug-ow reaction system.Prior to tests, the dried catalyst samples were ground, pelletized, crushed and sieved.Then, the pellets (0.1000 g) of size 0.22-0.45mm were loaded into a quartz tubular reactor (I.D. = 7 mm).Before ammonia synthesis, the catalysts were treated in H 2 (99.999%) ow at 573-873 K for 3 h to reduce ruthenium cations to a metallic state and to decompose CsNO 3 or Ba(NO 3 ) 2 .Ammonia synthesis reactions were carried out at 573 to 773 K and 0.10 MPa in a synthesis gas ow (H 2 /N 2 = 3/1, 99.999%, 60 mL min −1 ).The rate of ammonia formation was calculated based on the rate of decrease of the conductivity of H 2 SO 4 solution.During activity tests, aer the product gas had been stabilized under each reaction condition for 30 min, it was introduced into a sulfuric acid solution for measurement.

Characterization
The crystalline structure of samples was analyzed by X-ray diffraction (XRD) performed on an X'Pert Pro diffractometer (PANalytical) equipped with a Cu-Ka radiation source (l = 1.5405Å).All diffraction patterns were recorded in a 2q range of 10-70°at a scan speed of 2°min −1 .
The data for the N 2 adsorption-desorption isotherms collected at 77.3 K on an apparatus for automatic measurement of specic surface area and pore-size distribution (BELSORPmini II; Microtrac) were applied for evaluating the Brunauer-Emmett-Teller (BET) surface area of the prepared samples.The samples were degassed at 473 K under 10 −2 kPa for 2 h before the tests.The specic surface area was calculated from the linear part of the BET plot, where the P/P 0 ratios were 0.05-0.25.
The content of Nb and Ba in the products was detected by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) using the IRIS Intrepid II XSP system (Thermo Fisher Scientic).
The morphology of synthesized Ba 5 Nb 4 O 15 samples was observed by scanning electron microscopy (SEM) using a Zeiss system (Sigma) at an acceleration voltage of 20 kV.Images of Ru nanocrystallites on supports were recorded by transmission electron microscopy (TEM) on a JEM-2100 system (JEOL) at an acceleration voltage of 200 kV.
Temperature programmed desorption (TPD) tests were carried out on a self-built device, which was connected to a gas chromatograph (GC-2020; Trustworthy) equipped with a thermal conductivity detector (TCD).Before the desorption test, the catalyst (100.0 mg) was pretreated in N 2 (99.999%) ow of 15 mL min −1 at 773 K for 3 h, cooled down to 373 K, and exposed to a ow of 5 vol% NH 3 (balanced by 99.999% N 2 ) of 10 mL min −1 for 30 min to allow the sample to reach an adsorption equilibrium.Then, N 2 (99.999%) ow of 15 mL min −1 at 373 K for 30 min was used to remove the physically adsorbed ammonia molecules.In the desorption test, the catalyst was heated from 373 to 1073 K at 10 K min −1 .The desorbed ammonia was carried in N 2 ow (15 mL min −1 ) into the gas chromatograph for quantication.

Results and discussion
3.1 Effect of the Ba/Nb molar ratio on the morphology of barium-niobate samples Fig. 1 displays the XRD patterns of the samples of synthesized barium niobate.It can be seen that the molar ratio of Ba(OH) 2 / Nb 2 O 5 affected the formation of a crystalline structure of obtained materials.Ba 4 Nb 2 O 9 and Nb 2 O 5 phases (Fig. 1a) were observed in the sample while an equal amount of Ba(OH) 2 and Nb 2 O 5 (Ba/Nb = 0.5) was mixed and hydrothermally treated.By increasing Ba/Nb to 2.0, the other phases disappeared (Fig. 1b) and we could obtain a sample containing Ba 5 Nb 4 O 15 mainly in the orthorhombic phase (PDF#14-0028).Increasing Ba/Nb to 4.0 led to no apparent change in the crystalline structure of the samples (Fig. 1c).It can be inferred that the added Ba(OH) 2 exceeding the stoichiometry of Ba 5 Nb 4 O 15 had almost no impact on the crystalline structure of the resultant barium niobates.ICP-AES (Table 1) conrmed the XRD results that the measured Ba/Nb values agreed well with nominal Ba/Nb (added Ba/Nb) when the Ba/Nb ratio = 1.For Ba/Nb = 2 and 4, the measured Ba/Nb values were close to 1.2, which was consistent with the XRD pattern.These results indicated that excessive addition of Ba(OH) 2 was lost during ltration and waterwashing aer hydrothermal treatment.
Corresponding to their crystalline structures, elongated crystalline grains of size 200 × 1000 nm (Fig. 3a) could be obtained when the sample was prepared with a nominal Ba/Nb = 1.Increasing the addition amount of Ba(OH) 2 to Ba/Nb $2.0, we obtained Ba 5 Nb 4 O 15 akes, and the size of the akes decreased slightly with increasing the nominal Ba/Nb (Fig. 3b and c).These results indicated that excessive addition of Ba(OH) 2 accelerated the nucleation rate of niobates in hydrothermal synthesis and, as a result, aggregates of akes were formed.The 4 wt% Ru/Ba 5 Nb 4 O 15 catalyst prepared with a nominal Ba/ Nb = 2.0 gave the highest activity of 909 m mol g −1 cat h −1 .Further increase in the nominal Ba/Nb to >2.0 reduced the activity slightly (Fig. 4).Therefore, we chose Ba 5 Nb 4 O 15 prepared at Ba/ Nb = 2.0 as the support for Ru catalysts to study the effect of promoters.The acidity of the oxide support is detrimental to the activity of Ru catalysts for ammonia synthesis, and the surface acidity can be reected by TPD.The TPD prole (Fig. 2) suggested that the acid sites were weaker on a catalyst prepared with a nominal Ba/Nb = 2.0 than other catalysts, which was consistent with the results of the activity test.
The ammonia synthesis rate of different Ru loadings on Ba 5 Nb 4 O 15 is shown in Fig. 5. The-ammonia synthesis rate increased rst and then decreased with an increase in the addition amount of Ru, and reached a peak at 4 wt% Ru loading.Therefore, we chose 4 wt% Ru/Ba 5 Nb 4 O 15 to study the effect of promoters.
Table 3 lists the BET specic surface area, total pore volume and pore width of 4 wt% Ru/Ba 5 Nb 4 O 15 .Fig. 6 shows the adsorption/desorption isotherms of fresh and used 4% Ru/ Ba 5 Nb 4 O 15 .The BET specic surface area and pore volume of the fresh catalyst and used catalyst showed almost no change.The average pore size increased aer the reaction, possibly due to the detachment of impurities caused by a high temperature.

Catalytic activity of Ba-or Cs-promoted Ru/Ba 5 Nb 4 O 15
Tables 4 and 5 list the ammonia synthesis rates over 1Ba-and 2Cs-4 wt% Ru/Ba 5 Nb 4 O 15 activated in the synthesis gas (3H 2 + N 2 ) ow at different temperatures for 3 h.For both catalysts, the ammonia synthesis rate rst increased and then decreased when increasing the activation temperature.The optimal activation temperature for Ba-and Cs-promoted catalysts was 773 and 673 K, respectively.The activation process could transform the promoter into the oxide and/or hydroxide form.In addition, it could form and grow the Ru nanoparticle.A low activation temperature may not be sufficient to decompose the nitrate precursor of the promoter.However, sintering of supported Ru particles would occur and their particle size could increase at higher activation temperatures (e.g., 500 °C).Therefore, too low or high activation temperatures are not conducive to the formation of high performance.
Table 6 details the catalytic activity of Ru-based catalysts promoted with Ba(NO 3 ) 2 and CsNO 3 .It can be seen that all the promoters promoted the catalytic performance of the Ru/ Reaction conditions: prior to activity tests, the catalysts (100.0 mg) were reduced in H 2 flow of 15 mL min −1 at 573 K for 3 h; the activities were tested in 60 mL (STP) min −1 of synthesis gas (3H 2 + N 2 , 99.999%) at 673 K and 0.1 MPa.6 also compares the effect of different molar ratios of promoters.For the Ba-promoter, the ammonia synthesis rate was rst improved when the molar ratio of Ba/Ru was raised from 0 to 1, and then decreased when the molar ratio increased to 2. Therefore, the optimal molar ratio of Ba/Ru was 1, corresponding to ammonia synthesis of 3720 m mol g −1 cat h −1 at 400 °C and 0.1 MPa.For the Cspromoter, a molar ratio of Cs/Ru at 1 and 2 could both exhibit preferable activity for ammonia synthesis, but 2Cs/Ru could elicit higher activity at milder reaction conditions, and ammonia synthesis was 2363 and 4900 m mol g −1 cat h −1 at 573 K and 623 K, respectively.Both were 20-or 30-times higher than that of the unpromoted catalyst.Therefore, we postulated that 2Cs/Ru was the best loading ratio.A structural promoter can synthesize more B 5 active sites. 33,34In the 1Ba-4 wt% Ru/Ba 5 Nb 4 O 15 catalytic system, Ba is considered to be a structural promoter.This status leads to surface reconstruction of Ru and an increase in the number of B 5 active sites, thereby improving the performance of the catalyst.In the 2Cs-4 wt% Ru/Ba 5 Nb 4 O 15 catalytic system, Cs is considered to be an electronic promoter that can transfer electrons to the surface of active metals and reduce the dissociation energy of N 2 .In addition, barium in the carrier could have a synergistic effect with the Cs-promoter to improve the activity of ammonia synthesis signicantly.Fig. 7 shows the morphology of Ru nanoparticles on Ba 5 Nb 4 O 15 substrate aer reduction by exposure to H 2 and N 2 at 573 K for 3 h.Ru nanoparticles were evenly distributed on the surface of the Ba 5 Nb 4 O 15 support.The HRTEM image (Fig. 7b) shows hexagonal ruthenium particles uniformly deposited on the substrate.The lattice spacing of Ru particles was 0.211 nm, which corresponds to the (101) planes of Ru crystals. 35,36The lattice spacing of 0.293 and 0.309 nm corresponded to the (103) planes of Ru crystals and (110) planes of Ba 5 Nb 4 O 15 crystals, respectively. 37,38From the XRD spectrum of Ba 5 Nb 4 O 15 synthesized with Ba/Nb = 2.0, the (110) and (103) peaks were the toptwo most intensive diffractions, indicating that they were the preferred exposed facets of the Ba 5 Nb 4 O 15 akes.These exposed facets, consisting of oxygen anions, can induce the epitaxial growth of Ru nanocrystallites. 39It was demonstrated that the density of B 5 sites on the epitaxial growth of Ru was likely to be substantially greater than that for round particles.Fig. 7b reveals that the Ru particles seemed to be encapsulated, which explained why Ru/Ba 5 Nb 4 O 15 did not adsorb H 2 in the H 2 -pulse titration method.This phenomenon was attributed to the strong metal-support interaction (SMSI), which results in partial covering of Ru particles by the reduced support species. 40n order to explore the effect of reduction temperature on the size of ruthenium nanoparticles, over 300 ruthenium particles were measured from TEM images.Fig. 8 shows the size distribution of Ru particles at 573, 673 and 773 K; the average particle size of ruthenium was 2.48, 2.63 and 2.92 nm, respectively.The average particle size of ruthenium became larger as the temperature increased.Due to cover by Ba-or Cs-promoters (Fig. 9a and b) at room temperature, much fewer Ru particles were observed.However, they presented comparable sizes with those of Ru catalysts on Ba 5 Nb 4 O 15 .These Ru particles became exposed to the reactants at ammonia synthesis conditions. 41,42 showed a main peak at around 520 K. MgO over a similar temperature range split into two main peaks located at about 490 and 630 K.This result suggested that the acid sites were weaker on Ba 5 Nb 4 O 15 than on MgO.g-Al 2 O 3 and Sr 2 Nb 2 O 7 desorbed ammonia over a wide temperature range (400-1075 K), indicating that Ba 5 Nb 4 O 15 had stronger electron-donating ability and could provide electrons to ruthenium to improve the rate of ammonia synthesis.
4][45] Due to different Ru-loading, we calculated the ammonia synthesis rate based on Ru weight.The Ba-or Cspromoted Ru/Ba 5 Nb 4 O 15 catalyst exhibited a distinguished activity for ammonia synthesis, which was 10% higher than that of Cs-Ru/MgO, and 10-times higher than that of Ba-Ru/AC (industrial catalyst).Although Ru/C 12 A 7 :e−, Ru/La 2 Ce 2 O 7 , and

3. 2
Fig. 4 compares the ammonia synthesis rates over 4 wt% Ru catalysts supported on barium niobates synthesized with different nominal molar ratios of Ba/Nb at 673 K and 0.1 MPa.

Fig. 2
Fig.2Ammonia TPD profiles obtained over different molar ratios of Ba/Nb.Samples (100.0 mg) were exposed to 5 vol% NH 3 in N 2 flow (10 mL min −1 ) at 373 K for 30 min, and desorptions were performed from 373 to 1073 K at 10 K min −1 in N 2 carrier gas flow (15 mL).

Fig. 4
Fig.4Ammonia synthesis rate over a 4 wt% Ru catalyst supported on barium niobates prepared with different nominal molar ratios of Ba/ Nb.Reaction conditions: prior to activity tests, the catalysts (100.0 mg) were reduced in H 2 flow of 15 mL min −1 at 573 K for 3 h; the activities were tested in 60 mL (STP) min −1 of the synthesis gas (3H 2 + N 2 , 99.999%) at 673 K and 0.1 MPa.

Fig. 5
Fig. 5 Ammonia synthesis rate of different Ru loadings on Ba 5 Nb 4 O 15 .Reaction conditions: prior to activity tests, the catalysts (100.0 mg) were reduced in H 2 flow of 15 mL min −1 at 573 K for 3 h; the activities were tested in 60 mL (STP) min −1 of synthesis gas (3H 2 + N 2 , 99.999%) at 673 K and 0.1 MPa.

Table 1
ICP-OES results of Nb and Ba content in different Ba/Nb hydrothermal products at 473 K for 24 h

Table 3
BET specific surface area, total pore volume (V p ), and pore width of 4 wt% Ru/Ba 5 Nb 4 O 15

Table 6
Ammonia synthesis rate over 4 wt% Ru/2.0BaNb (Ba 5 Nb 4 O 15 ) catalysts promoted by barium or cesium a Values before the promoter in the catalyst names are the molar ratios of Ba or Cs to Ru.

Table 7
Comparison of ammonia synthesis activity over Ru catalysts supported on different substrates

Table 8
BET specific surface area of the 1Ba or 2Cs-4 wt% Ru/ Ba 5 Nb 4 O 15 catalyst measured at different phases